Acoustic responses measured in the ear-canals of infants vary significantly at ages 1- to 24-months relative to adult responses. such external- and middle-ear factors in development have significance for clinical detection of hearing impairment, and the interpretation of experiments on otoacoustic emissions, ABR, behavioral responses, and neural development. Most of these areas have been separately investigated in the previous Program Project. It is proposed to link responses using multiple measurement techniques on groups of infant and adult subjects. The measured data will be used to estimate parameters in theoretical models of the external and middle ear, to model the inner ear using existing cochlear models as well as a new active cochlear model, and to combine these models with the external- and middle-ear models, thereby obtaining a quantitative developmental model of the auditory periphery. The parameters in a model of auditory peripheral development (external, middle, and inner ear) will be estimated based upon impedance and reflection coefficient measurements in infants and adults, and model will be used to predict and interpret data from experiments on hearing development. A new active model of cochlear micromechanics will be analyzed that combines a nonlinearly saturating hair-cell stiffness with phase delay from a resonant tectorial membrane. In four different collaborations within the Program Project, impedance and reflection coefficient measurements will be carried out on groups of infants, including those with normal and abnormal auditory systems, who will be tested also for otoacoustic emissions, and ABR and behavioral thresholds. The impedance and reflection coefficient method has potential clinical advantages over tympanometry as a tool for detecting middle-ear impairment in neonates. The proposed studies will enable these advantages to be assessed. The results from these studies will provide information on the extent to which external-and middle-ear development accounts for the differences observed between infants and adults in otoacoustic emission levels and frequency ranges, ABR and behavioral thresholds, and measures of hearing on infants with an abnormal auditory system.